Laser light producing devices have a myriad of applications including communications, guidance systems, surveying, emergency rescue, and military applications. Current laser systems require complex, expensive, and fragile electronic circuits, components, and batteries, or other electronic power sources. Metals are required in the construction of these systems and are susceptible to detection due to this metallic content. These systems also produce residual electrical emissions that could interfere with other electronic/electrical systems.
Vertical cavity surface emitting lasers (VCSELs) based on inorganic semiconductors (e.g. AlGaAs) have been developed since the mid-80's (Susumu Kinoshita et al., IEEE Journal of Quantum Electronics, Vol. QE-23, No. 6, June 1987, pages 882-888). They have reached the point where AlGaAs-based VCSELs emitting at 850 nm are manufactured by a number of companies and have lifetimes beyond 100 years (Kent D. Choquette et al., Proceedings of the IEEE 85, Vol. 85, No. 11, November 1997, pages 1730-1739). With the success of these near-infrared lasers, attention in recent years has turned to other inorganic material systems to produce VCSELs emitting in the visible wavelength range (C. Wilmsen et al., Vertical-Cavity Surface-Emitting Lasers, Cambridge University Press, Cambridge, 2001, pp. 268-276). There are many potential applications for visible lasers, such as, display, optical storage reading/writing, laser printing, and short-haul telecommunications employing plastic optical fibers (T. Ishigure et al., Electronics Letters, Vol. 31, No. 6, Mar. 16, 1995, pages 467-469). In spite of the worldwide efforts of many industrial and academic laboratories, much work remains to be done to create viable laser diodes (either edge emitters or VCSELs) that produce light output that spans the visible spectrum.
In an effort to produce visible wavelength VCSELs it would be advantageous to abandon inorganic-based systems and focus on organic-based laser systems, since organic-based gain materials can enjoy a number of advantages over inorganic-based gain materials in the visible spectrum. For example, typical organic-based gain materials have the properties of low unpumped scattering/absorption losses and high quantum efficiencies. In comparison to inorganic laser systems, organic lasers are relatively inexpensive to manufacture, can be made to emit over the entire visible range, can be scaled to arbitrary size and, most importantly, are able to emit multiple wavelengths (such as red, green, and blue) from a single chip. Finally, organic lasers have a very large gain bandwidth, especially in comparison with inorganic lasers. Over the past number of years, there has been increasing interest in making organic-based solid-state lasers. The laser gain material has been either polymeric or small molecule and a number of different resonant cavity structures were employed, such as, microcavity (see Kozlov et al., U.S. Pat. No. 6,160,828 issued Dec. 12, 2000, titled “Organic Vertical-Cavity Surface-Emitting Laser”), waveguide, ring microlasers, and distributed feedback (see also, for instance, G. Kranzelbinder et al., Rep. Prog. Phys. 63, (2000) pages 729-762 and M. Diaz-Garcia et al., U.S. Pat. No. 5,881,083 issued Mar. 9, 1999, titled “Conjugated Polymers As Materials For Solid State Laser”). A problem with all of these structures is that in order to achieve lasing it was necessary to excite the cavities by optical pumping using another laser source. It is much preferred to electrically pump the laser cavities since this generally results in more compact and easier to modulate structures.
A main barrier to achieving electrically pumped organic lasers is the small carrier mobility of organic material, which is typically on the order of 10−5 cm2/(V−s). This low carrier mobility results in a number of problems. Devices with low carrier mobilities are typically restricted to using thin layers in order to avoid large voltage drops and ohmic heating. These thin layers result in the lasing mode penetrating into the lossy cathode and anode, which causes a large increase in the lasing threshold (see V.G. Kozlov et al., Journal of Appl. Physics, Vol. 84, No. 8, Oct. 15, 1998, pages 4096-4108). Since electron-hole recombination in organic materials is governed by Langevin recombination (whose rate scales as the carrier mobility), low carrier mobilities result in orders of magnitude having more charge carriers than singlet excitons; one of the consequences of this is that charge-induced (polaron) absorption can become a significant loss mechanism (N. Tessler et al., Applied Physics Letters, Vol. 74, No. 19, May 10, 1999, pages 2764-2766). Assuming laser devices have a 5% internal quantum efficiency, using the lowest reported lasing threshold to date of ˜100 W/cm2 (M. Berggren et al., Nature, Vol. 389, Oct. 2, 1997, pages 466-469), and ignoring the above mentioned loss mechanisms, would put a lower limit on the electrically-pumped lasing threshold of 1000 A/cm2. Including these loss mechanisms would place the lasing threshold well above 1000 A/cm2, which to date is the highest reported current density, which can be supported by organic devices (Nir Tessler, Advanced Materials, 1998, 10, No. 1, pages 64-68).
One way to avoid these difficulties is to use crystalline organic material instead of amorphous organic material as the lasing media. This approach was recently taken (J. H. Schon, Science, Vol. 289, Jul. 28, 2000, pages 599-601) where a Fabry-Perot resonator was constructed using single crystal tetracene as the gain material. By using crystalline tetracene, larger current densities can be obtained, thicker layers can be employed (since the carrier mobilities are on the order of 2 cm2/(V−s)), and polaron absorption is much lower. Using crystal tetracene as the gain material resulted in room temperature laser threshold current densities of approximately 1500 A/cm2.
An alternative to electrical pumping for organic lasers is optical pumping by incoherent light sources, such as, light emitting diodes (LEDs), either inorganic (M. D. McGehee et al., Applied Physics Letters, Vol. 72, No. 13, Mar. 30, 1998, pages 1536-1538) or organic (Berggren et al., U.S. Pat. No. 5,881,089 issued Mar. 9, 1999, titled “Article Comprising An Organic Laser”). This possibility is the result of unpumped organic laser systems having greatly reduced combined scattering and absorption losses (˜0.5 cm−1) at the lasing wavelength, especially when one employs a host-dopant combination as the active media. Even taking advantage of these small losses, the smallest reported optically pumped threshold for organic lasers to date is 100 W/cm2 based on a waveguide laser design (M. Berggren et al., Nature 389, Oct. 2, 1997, pages 466-469). Since off-the-shelf inorganic LEDs can only provide up to ˜20 W/cm2 of power density, it is necessary to take a different route to enable optically pumping by incoherent sources. Additionally, in order to lower the lasing threshold it is necessary to choose a laser structure that minimizes the gain volume.
Undesirable electrical noise can be emitted from LEDs and require an external power supply, along with electronic components/circuitry to operate. These circuits are usually metallic and can be detected in covert situations. Damage to these LEDs and supporting circuitry can occur from external electromagnetic emissions.
What is needed is a reliable, robust, laser light producing device capable of producing a laser emission in extreme environments without the previously mentioned limitations of conventional electrical sources and supporting circuitry that power the laser light producing device.